Fierce fighting has taken place in a town near the Syrian capital that a UN team visited earlier this week to probe an alleged chemical weapons attack, anti-government activists say.

"Violent fighting is pitting regular armed forces against rebels on the northern and western fronts of Moadamiyet al-Sham," the Britain-based Syrian Observatory for Human Rights said.

"The army is trying to advance into the town," it said, adding that an air raid and a surface-to-surface missile launched by the army had targeted the area southwest of Damascus.

Moadamiyet al-Sham is one of the places allegedly hit by chemical attacks on August 21 that the opposition say killed hundreds, and UN inspectors who are currently probing if the incident actually took place visited the town on Monday.

According to the Observatory, the town was hit by more than 10 air raids and hundreds of missiles and rockets on the day of the alleged gas attack.

The army has for months been trying to take Moadamiyet al-Sham and Daraya - another Damascus suburb - back from the rebels, without success.

According to Damascus-based anti-regime activist Ibrahim Shaiban, these two rebel strongholds have been "intensely" bombed since Thursday.

"Regime forces have also dispatched men and tanks as back-up," he said. On Thursday, eight people - including at least one woman and a child - died in Moadamiyet al-Sham bombings, and one person was killed in Daraya.

A top security official in Damascus said the army was "in permanent confrontation with terrorist groups who are in Moadamiyet, and have been for a long time".

The UN team is currently on the final leg of its Syria probe, and on Friday was heading to a military hospital in a Damascus neighbourhood, another security official said.

They are due to leave Syria by Saturday morning, and will report straight back to UN head Ban Ki-moon.